1. Field of the Invention
The present invention relates to a furnace for heating a glass preform for an optical fiber and a method for producing such glass preform. More particularly, it relates to a heating furnace and a method for heating a porous glass preform consisting of fine particles of quartz glass to thermally treat. The heating furnace of the present invention can prevent contamination of the glass preform with impurity elements and has good durability.
The term "thermally treat" is intended to mean that the glass preform is heated in a muffle tube which is installed inside a heater element to separate an heating atmosphere from the heater element so that the preform is dehydrated, fluorine-added and/or vitrified.
2. Description of the Related Art
As one of general methods for mass producing a glass preform for use in the fabrication of an optical fiber, the VAD (Vapor Phase Axial Deposition) method is known. The VAD method comprises depositing fine particles of glass generated in oxyhydrogen flame on a rotating starting member such as a glass plate or rod to form a cylindrical porous preform (soot preform) and sintering said porous preform to obtain a transparent glass preform for use in the fabrication of the optical fiber.
In the VAD method, for sintering the porous preform to covert it into transparent glass, the preform should be heated in an atmosphere of an inert gas (e.g. helium and argon) to a temperature of 1,600.degree. C. or higher. As a heating furnace for sintering the preform, usually a heating furnace having a carbon heater is used. What should be taken care of when sintering the preform in such heating furnace is inclusion of transition metals such as copper or iron and water. When 1 (one) ppb or larger of the transition metal is included in the glass preform, transmission loss wavelength characteristics of the fabricated optical fiber is greatly deteriorated in an entire wavelength range. When 0.1 ppm or larger of water is included in the preform, the characteristics of the fabricated optical fiber is impaired in a longer wavelength range.
Therefore, the porous preform is usually dehydrated before or during vitrification. As a dehydration method, it is known to heat the porous preform at a high temperature in an atmosphere of an inert gas containing a chlorine-containing gas. When the porous preform is heated at a high temperature in an atmosphere of an inert gas containing a fluorine-conatining gas, fluorine is added to the porous preform. When the fluorine is added to the porous preform, a refractive index profile which is essential to the optical fiber is advantageously adjusted. In this connection, Japanese Patent Publication No. 15682/1980 and Japanese Patent Kokai Publication No. 67533/1980 can be referred. These publications will be discussed below.
The treatment with the fluorine-containing gas is carried out in the heating furnace before or simultaneously with vitrification. To prevent wastage of the carbon heater due to moisture or oxygen which is generated during heating of the preform, a muffle tube is installed for separating the carbon heater and the sintering atmosphere. As the muffle tube, a quartz glass made one is conventionally used.
Japanese Patent Publication Nos. 58299/1983 and 42136/1983 and Japanese Patent Kokai Publication No. 86049/1985 disclose the use of the quartz glass made muffle tube in detail.
The fluorine-containing gas is decomposed or reacts at a high temperature to form F.sub.2 gas or HF gas. These gases react with the quartz glass according to the following reaction formulae to generate SiF.sub.4 gas, and by these reactions, the quartz glass is etched: EQU SiO.sub.2 +2F.sub.2 .fwdarw.SiF.sub.4 +O.sub.2 EQU SiO.sub.2 +4HF.fwdarw.SiF.sub.4 +2H.sub.2 O
Because of such etching, copper and iron present inside the quartz glass appear on the surface and contaminate the porous preform. In addition, by etching, pin holes are formed in the quartz made muffle tube, which is a cause of intake of environmental air or leakage of the atmosphere in the muffle tube. These are not advantageous for the production method.
Furthermore, the quartz glass tube has a very bad problem that it tends to easily deform at a high temperature. That is, when the quartz glass is kept at about 1,300.degree. C. for a long time, it deforms due to viscous flow. In addition, when it is used at a temperature of 1,150.degree. C. or higher for a long time, it is devitrified, and once the furnace temperature is lowered, strain is generated due to difference of thermal expansion coefficient between the glass phase and the devitrified phase and finally breaks the tube.
As a material which hardly reacts with the fluorine-containing gas or the chlorine-containing gas, carbon is contemplated. The carbon does not either react with SF.sub.6, C.sub.2 F.sub.6, CF.sub.4 and the like which easily react with the quartz. Of course, the carbon does not react with SiF.sub.4.
Japanese Patent Publication No. 28852/1981 suggests the use of a carbon made muffle tube in an atmosphere comprising the fluorine-containing gas such as F.sub.2, although no working example is described.
However, the carbon has following drawbacks:
1. Since the carbon has minute pores, gases can penetrate therethrough. Permeability of nitrogen through the carbon is 10.sup.6 times larger than through the quartz glass.
2. The carbon is easily oxidized and, at a temperature not lower than 400.degree. C., it easily reacts with oxygen to form CO.sub.2 or CO.
To prevent oxidation, it has been proposed to form a layer of ceramics such as SiC, Al.sub.2 O.sub.3 and BN on an inner wall of the carbon muffle tube. Although the ceramics layer prevents the oxidation, it disadvantageously reacts with at least one of the chlorine-containing gas and the fluorine-containing gas. Impurities generated by such reaction devitrify the soot preform and generate bubbles in the soot preform.
Since the carbon is a material having large gas permeability as described above, the gas goes in and out through the wall of the muffle tube so that the moisture in the air penetrates into the muffle tube through the wall. Therefore, the glass preform contains a comparatively large amount of water and in turn the hydroxyl groups. In addition, the gasses such as Cl.sub.2 and SiF.sub.4 are released outside the furnace through the wall and may pollute a work environment, and impurities (e.g. copper and iron) may penetrate into the furnace from the outside. These defects can be considerably overcome by increasing the thickness of carbon, but still not completely.
As explained above, the addition of fluorine to the quartz glass of the cladding part by the conventional methods encounters various difficulties.